1. Field of the Invention
This invention generally relates to elastic wave devices and package substrates, and more particularly, to an elastic wave device such as a SAW device or a FBAR device and a package substrate having the elastic wave device thereon.
2. Description of the Related Art
The elastic wave device such as the SAW device or the FBAR device is a small-sized and inexpensive device, and is used in a wide range of applications, for example, a bandpass filter on a mobile telephone. The SAW (Surface Acoustic Wave) device utilizes Rayleigh waves that travel on a surface of an elastic body. The FBAR (Film Bulk Acoustic Resonator) device utilizes vibrations of a piezoelectric film. These elastic wave devices are essential for downsizing communications devices as represented by the mobile telephone, and are demanded to be downsized in order to meet the requirements of further downsized mobile communications terminals in these years.
A size of a substrate is one factor that determines the whole size of the elastic wave device. The size of the substrate denotes the size of the substrate for forming the device before the device is mounted on the package. This size largely depends on the arrangement of excitation electrodes (for exciting the elastic waves) provided on the substrate, the device design such as frequency when used as a device, or the physical characteristics of a material of the substrate having the excitation electrode thereon. For example, the piezoelectric substrate is used for the SAW device. The velocity of the SAW depends almost entirely on the physical characteristics of the piezoelectric substrate. The velocity of the surface wave (v), a gap between the excitation electrodes (period: π), and the frequency of the device (f) has a relationship denoted by v=f×λ. Therefore, the package on which the device is mounted, other than the substrate on which the device is formed, has to be downsized in order to downsize the elastic wave device as a whole.
A description will be given of a structure of the package for the conventional elastic wave device.
FIGS. 1A and 1B illustrate a package structure of the SAW device mounted in a facedown state. FIG. 1A is a bottom view of the piezoelectric substrate mounted on the package. FIG. 1B is a cross-sectional view of the SAW device having the piezoelectric substrate mounted thereon in the facedown state. A bottom surface of a piezoelectric substrate 101 includes a pair of comb-like electrodes 102a (the excitation electrodes) having reflection electrodes 103a and 103b on both sides thereof and another pair of comb-like electrodes 102b (the excitation electrodes) having reflection electrodes 103c and 103d on both sides thereof. The above-mentioned excitation electrodes are respectively connected to signal pads 104a, 104b, and 104c provided on the piezoelectric substrate 101.
Two layers of an upper substrate 105a and a lower substrate 105b are deposited to form the package substrate on the bottom of a package 105. The upper substrate 105a is mounted on the lower substrate 105b on which footpads 106 are formed. The footpads 106 establish electric connection to the outside. A top surface of the upper substrate 105a is a die attach surface 109, and package side signal pads 110 (simply referred to as signal pads) are provided on the die attach surface 109. A stud bump 111 made of gold is provided on the side of the piezoelectric substrate 101, and is connected to the package side signal pad 110.
The piezoelectric substrate 101 is housed in the package 105 in the facedown state so that a main surface having the excitation electrodes thereon may face the die attach surface 109. The signal pads 104a, 104b, and 104c are respectively coupled to the stud bumps 111 by flip-chip bonding so as to connect the corresponding package side signal pads 110. The package side signal pads 110 are coupled to the footpads 106 provided on the lower substrate 105b, which is the backside of the package 105, through an interconnection. An open region of thus configured package 105 is covered with a metal cap 107, and is hermetically sealed with a seal 108. Generally, the seal 108 is heated and melted to hermetically seal the package 105 and the cap 107. The seal 108 employs a solder having a high-melting point, for example, AuSn solder, so as to stand up with a normal reflow temperature. Here, AuSn solder denotes a solder alloy including 80% of Au and 20% of Sn.
A ceramics such as alumina is generally used for the package 105. If alumina ceramics is used for the package 105, an interconnection pattern is printed with the use of tungsten (W) paste, is baked, and is plated with Ni, Au, or both. Generally, the interconnections are thus formed on the upper substrate 105a and the lower substrate 105b. 
In these years, a CSP (Chip Size Package) has been developed to further downsize the package for the SAW device. Japanese Patent Application Publication No. 2002-513234 (hereinafter referred to as Document 1) and Japanese Patent Application Publication No. 2000-77970 (hereinafter referred to as Document 2) disclose the package in which the piezoelectric substrate is provided above the plate (also referred to as a mount substrate) serving as a base so as to have a gap between the substrate and the plate, and the circumference of the package is hermetically sealed with a sealing material. The aforementioned package structure requires a highly hermetic sealing material in order to prevent the excitation electrodes provided on the piezoelectric substrate from degrading due to moisture or gas. A metallic material is effective for the sealing material.
FIGS. 2A, 2B, 3A, and 3B illustrate the package structure of the above-mentioned CSP package. FIGS. 2A and 3A are bottom views of piezoelectric substrates mounted on the packages. FIGS. 2B and 3B are cross-sectional views of the SAW devices on which the piezoelectric substrates shown in FIGS. 2A and 3A are mounted in the facedown state. FIG. 4A is a bottom view of an FBAR substrate mounted on the package. FIG. 4B is a cross-sectional view of the FBAR device on which the FBAR substrate shown in FIG. 4A is mounted in the facedown state. Here, the FBAR substrate includes a piezoelectric thin film arranged in a region where the FBAR chip is to be formed. For instance, a piezoelectric thin film layer is provided on a silicon substrate, and the excitation electrodes are formed on the aforementioned piezoelectric thin film layer. In FIGS. 2A through 4B, the same components and configurations as those of FIG. 1 have the same reference numerals. The substrate for the elastic wave device (such as the above-mentioned piezoelectric substrate or the FBAR substrate) collectively denotes the substrate on which the elastic wave device such as the SAW device or FBAR device is formed. The substrate for the elastic wave device has to be distinguished from the package substrate.
In FIGS. 2A through 4B, there are provided a substrate side sealing electrode (referred to as a second sealing electrode) 112, a package side sealing electrode (referred to as first sealing pad or sealing pad) 113, a sealing material 114, and a ground terminal 115. In FIG. 4A, an upper excitation electrode of the FBAR is indicated by reference numeral 116, a lower excitation electrode of the FBAR is indicated by reference numeral 117, and an FBAR substrate is indicated by reference numeral 101′. The SAW device shown in FIGS. 2A and 2B is configured so that the package side sealing electrodes 113 are not coupled to the ground terminal 115. However, the SAW device shown in FIGS. 3A and 3B is configured so that the package side sealing electrodes 113 are coupled to the ground terminal 115 inside the package substrate.
The piezoelectric substrate 101 mounted in the facedown state is, in many cases, configured to employ sheet-shaped substrates for the upper substrate 105a and the lower substrate 105b provided on the bottom of the package 105. In the aforementioned case, several tens to several hundreds of piezoelectric substrates 101 are arranged on the sheet-shaped substrate, are sealed with the metal cap 107, and are diced into the respective elastic wave devices from a side of the sheet-shaped substrates.
The CSP has a structure that normally employs the paste printing or metal plating so as to form the sealing material on the side of the package with a metallic material. When the paste printing is employed, the pattern having a narrow line width cannot be formed without downsizing the grain diameter of the paste. Moreover, a highly accurate positioning is required. Generally, the grain diameter of the paste used for the paste printing has to be approximately ⅓ to ⅕ of the line width of the pattern. However, the currently available technique is just capable of decreasing the grain diameter of the paste to approximately 15 μm. This results in a printable line width of 45 to 75 μm at most. As described above, the package size of the elastic wave device is demanded to be downsized so that the elastic wave device can be downsized as a whole. For this purpose, the package side sealing electrodes 113 have to be narrowed; however, the above-mentioned line width is not enough.
On the other hand, when the metal plating is employed for forming the sealing material, a mask formed by a photo litho process is used. With this mask, the fine processing can be performed easily. It is thus easy to narrow the line widths of the package side seal electrodes. It is to be noted that a series of photo litho process includes multistage processes of resist coating, baking, exposure (mask forming), development, baking, plating, and resist removal. This requires time and cost. Moreover, the package substrate employs the alumina ceramics or glass ceramics that expands and contracts, the contraction percentage differs depending on the package substrates to be used. There arises a problem in that the positioning is misaligned in the mask forming process and the pattern is also misaligned.